Bill Gates has conceded that his “greatest mistake ever” was failing to put Microsoft in Android’s position as the world’s biggest “non-Apple” mobile operating system. The Microsoft co-founder was reflecting on the era when the market was shifting toward mobile, in an interview at venture capital firm Village Global.

“In the software world, particularly for platforms, these are winner-take-all markets. So the greatest mistake ever is whatever mismanagement I engaged in that caused Microsoft not to be what Android is. That is, Android is the standard non-Apple phone platform. That was a natural thing for Microsoft to win,” he said.

“It really is winner take all. If you’re there with half as many apps or 90% as many apps, you’re on your way to complete doom. There’s room for exactly one non-Apple operating system and what’s that worth? $400 billion that would be transferred from company G to company M.”

In 2017, Gates said he used an Android phone, but didn’t specify which one. The Gates Foundation (which Bill founded with wife Melinda) didn’t immediately respond to a request for comment on whether that was still the case.

It’s also not the first goof Gates has admitted to. In 2013, he said that the control-alt-delete function, which allows users to log in to Windows and access the task manager, was a mistake that came after an IBM keyboard designer wouldn’t give them a single button for it.